I've been looking to the new features of 11.0.8 and tried to find out what the drawing mode "B color to A color" is doing. Hmmm? It is doing nothing. It is not drawing in A color, not in B color. It is not drawing at all. I expectet something like mixing between B and A.

1. Draw something with A color.
2. Set the A color to be the B color.
3. Set the tools drawing mode to "B color to A color"
4. Choose a new A color
5. Draw over the first drawing
6. The strokes of the first drawing will change their color (which is now the B color) to the new A color

Yes, the idea is to change only the color B into color A, and leave the other surrounding colors untouched.
However, it works only with colors with full opacity and power, be careful not to use AAliasing for that matter.